1. Field of the Invention
This invention relates to a data transmission system and method, and more particularly to a looped data transmission system and a method using a polling signal.
2. Description of the Prior Art
It is often required to exchange data among a large number of stations. In such case, a looped data highway system is frequently adopted. In the looped data highway system, the transmission stations are successively connected in cascade by a serial data transmission line. Accordingly, data needs to be correctly transmitted and received in a short time between the transmission station sending out the data and the transmission station receiving the data. Further, when two or more transmission stations request to send out data at the same time, they need to be collectively controlled so as to prevent collision of data on the transmission line.
Therefore, a polling system has heretofore been adopted. By way of example, one of the stations of the looped data highway system is determined as a central station for purposes of control. The central station sends out to one adjacent transmission station a polling signal which allows it to send out data. When this transmission station has no data to be sent out, it sends out the polling signal to the adjacent transmission station. When any of the transmission stations receiving the polling signal successively sent out in this manner is the station requesting the transmission of data, this station inhibits the polling signal from being sent out to the next station and sends out a data signal which consists of the transmission data, address data indicative of the station itself and address data indicative of the transmission station to receive the transmission data. The adjacent transmission station receives the data signal sent out, and when it is not the station to receive this data signal, it transfers it to the adjacent transmission station. The transmission station appointed by the reception address of the data signal thus transferred receives and loads the data signal. The station having received and loaded the data signal sends out a response signal which consists of its own address, the address of the station having sent out the received data signal, and data to the effect that the data signal has been received. The response signal is successively transferred among the transmission stations similar to the data signal, and is received and loaded into the transmission station which has sent out the data signal. When the data to be transmitted and received have been correctly transmitted and received in this way, the transmission station which has sent out the data signal and which has confirmed the correct reception thereof by the reception of the response signal sends out the polling signal to trigger the next transmission and reception of data. When the response signal is not received, this polling signal is not sent out. At this time, the central station monitors a time interval from the sending-out of the first polling signal to the sending-out of the next polling signal, and if the time interval exceeds a predetermined value, the central station sends out the polling signal so as to prevent the system from stagnating due to the bad condition of the first transmission and reception. However, a problem arises in a case where the data sending-out request is made by a transmission station which lies upstream of, or closer to the central station than, the transmission station which has responded to the first polling signal of the central station. In this case, the station having responded to the first polling signal maintains the data signal sending-out state, and also the upper stream station having received the next polling signal falls into the data signal sending-out state.
When the two or more stations have been simultaneously placed in the data signal sending-out states in the looped data highway system, this system locked-up and becomes incapable of transmitting or receiving data.